This research was begun to examine the biophychosocial development of adolescents, particularly to identify the factors leading to poor adjustment and mental health. Investigating why girls, more than boys, develop mental illness during adolescence is of special interest. This long-term objective continues with the proposed research. The proposed studies have three specific aims: (1) to further establish the importance of the early adolescent transition for mental health by examining the longer term consequences four years later, (2) to establish the generalizability of the model by replicating the basic findings in a second population, and (3) to verify the hypothesized phenomena by attempting to alter the developmental course through an intervention. The major hypothesis is that the challenges of the early adolescent transition cause divergence in developmental trajectories, with some individuals developing increasingly poorer mental health over adolescence. The first aim will be addressed by analyzing the follow-up data just collected to examine the continuity or discontinuity of mental health status from early adolescence to the end of middle adolescence. The predictors of negative mental health outcomes later in adolescence and of positive as compared to negative developmental pathways will be identified. Gender differences in developmental patterns and processes will continue to be of particular interest. Finally, although puberty does not appear to affect the mental health of all adolescents, further analyses will identify the salient characteristics of those who are affected by pubertal change. The second major specific aim will be addressed by replicating the important results on the development of mental health with a second sample, one similar in socioeconomic status but varying in geographical region. The replication study will also provide baseline data for use in the third study. In the third study, we will examine micro processes of coping with change and will intervene with the development of serious problems among young adolescents, to ameliorate the negative developmental trends. Two standard intervention models, modified to address the specific areas identified in the earlier research, will be tested as young adolescents make the transition to junior high school. A subset of common measures will be used in all three studies. This research will advance our understanding of the developmental processes leading to mental health as compared to mental illness during adolescence, the phase of life in which most psychological difficulties seen in adulthood first become manifest. Developmental processes leading to differential susceptibility of girls relative to boys will be particularly examined.